Studies of natural maintenance mechanisms for La Crosse encephalities arbovirus in forested areas in the large endemic region in rural southwestern Wisconsin and in foci of virus activity as recently found in suburban eastern Wisconsin will be continued. Endemic and non-endemic areas will be further delineated by following distribution of cases of California encephalitis group arboviruses, and by isolates from humans, animals and mosquitoes. Transovarial, venereal, and oral transmission will be further studied using field strains of La Crosse virus and Aedes triseriatus mosquitoes collected from endemic and non-endemic areas. Old auto and tractor tires will be evaluated as developing main sources of transovarially infected mosquitoes around cases and as likely means of spread of virus from endemic to non-endemic areas. Control efforts for La Crosse virus employing reduction of vector ovi-position sites in basal tree-holes and old tires will be evaluated in La Crosse County and in isolated endemic foci. Lower rates of venereal infection and transmission observed in females mated after previous engorgement on chipmunks with antibody will be studied as a possible natural control mechanism.